This invention relates generally to a sheet handling system for transporting sheets to and from a transfer station and, more particularly, concerns a sheet handling system for transporting copy sheets to and from a transfer station for out-of-sequence imaging and later integration to form a stream of imaged copy sheets in a predetermined collated order.
U.S. Pat. No. 5,095,342, issued to Farrell et al., assigned to the Xerox Corporation and issued on Mar. 10, 1992, is hereby incorporated herein by reference thereto.
Sheet transports are well known, and, generally, have a defined path through which sheet material is transported to and from one or more process stations. In electrophotographic printing machines, such as copiers and printers, as well as other printing machines, sheet transports are employed to transport sheet materials, generally known as copy sheets, to and from a processing station to impart or imprint an image to or on the copy sheets. For example, in electrophotographic printing machines, sheet transports transport or carry sheet material through a transfer station where a developed image is transferred to the sheet material.
Copy sheet transports of the type to which this invention relates transport a variety of sheet material (e.g., paper, Mylar, and the like) from an input area, which generally is a tray or bin in which a stack of sheets are located, along a defined path. The path generally includes processing stations for imprinting and fixing an image on the transported sheets and apparatus for directing the image bearing sheet toward a sheet output station. Copy sheet transports used with electrophotographic printing machines which print plural discrete images on one copy sheet frequently are provided with a recirculating mode whereby copy sheets are fed from a tray and passed to a transfer station and a fusing station and then, in sequence, to a buffer or holding tray for subsequent recirculation to the transfer station for duplex printing.
Other printing machines, which print multiple discrete images on individual copy sheets, have a recirculation path for such operation so that copy sheets after imaging move along the path to return to the imaging station without a holding tray. Still other printing machines provide a composite image which is formed by plural passages of the imaging surface so that plural images are formed on the imaging surface and transferred to the copy sheet. Many sheet transports also invert the copy sheets prior to return to the transfer station so that the recirculated copy sheets may be imaged on both sides (i.e., duplexing). In any event, the prime function of sheet transports of the type to which this invention relates is to transport copy sheets from a stack along a path through a transfer station and, at the fusing station, and onto an output or finishing station in a predetermined collated order.
In general, printing machines are of two types, precollation and post-collation. Precollation devices generally include a printing machine which produces copy sheet sets seriatum until the desired sets "N" are made. The Docutech model printing system available from Xerox Corporation and other machines similarly operating use essentially a hybrid precollation method of producing concurrently two streams of collated sheets intermixed which are separated prior to two stacking trays by an alternatively positionable gate. These machines operate in contradistinction to post-collation-type devices where sheets are output page 1-1.sub.n, 2-2.sub.n, etc. where "N" is the number of sets desired or the sorter capacity of the device. The present invention, while employable with both precollation and post-collation devices, is particularly useful in the precollation and hybrid precollation devices.
The devices with which copy sheet transports are most frequently associated are often given tasks or jobs (e.g., to image a series of copy sheets) which require transport of copy sheets. Often in performing such jobs on one type of printing machine which employs plural cycling of the printed copy sheets, some of the copy sheets transported require less cycles than are required for other copy sheets being printed (e.g., a black and white copy sheet in a two pass highlight color printer needs only one pass compared to copy sheets printed in black and highlight color. Likewise, a simplex copy sheet, a copy sheet printed on one side, needs only one pass compared to the duplex copy sheet, and so on). Thus, when printing sheets requiring multiple passes and sheets requiring fewer passes, present copy sheet transports cycle all sheets through the machine unless other arrangements are made, such as, for example, running the jobs separately and then manually collating the separate jobs
A similar problem also arises in printing machines in which the imaging surface is cycled a plurality of times to create a composite image, generally an image composed of several images such a color images, for transfer. In this case, jobs can contain certain composite images which require fewer cycles than other composite images. For example, one image may require only one cycle while a two color image requires two cycles of the imaging surface, so that where a two color image is followed by a black image, pitches can be skipped in copy sheet transports which employ precollation techniques. This, like the aforementioned plural pass printing machines which required cycling of sheets which are not imaged, the cycling of completed images on the imaging surface is wasteful of time and resources. Both types of machines, those which cycle sheets and those which cycle the imaging surface a plurality of times to create a composite image, will be collectively referenced hereinafter as plural cycle printing machines.
To restate this, copy sheet transports used with plural cycle printing machines of the type in which successive images were transferred to the copy sheet often present sheets for printing at the transfer station which are not imaged (e.g., a simplex document contained within a job having duplex documents, a black and white image document in a job having multi-color images). Also, copy sheet transports used with plural cycle printing machines of the type in which a composite image is formed on a surface and then transferred to a copy sheet, often skip pitches when jobs contain composite images requiring different numbers of cycles. Needless to say, a single such waste is unnoticed, but when many such documents are in the job, it is an inefficient construction, wasting both resources and machine time. The following disclosures may be relevant to various aspects of the present invention: